The present method of assessing the useful life of pressure tubes in CANDU reactors requires the periodic removal of a tube. Samples are cut from the removed tube and analyzed for deuterium content. The deuterium concentration is then used as a measure of the useful life of the remaining pressure tubes. This approach is very costly because of the long shutdown period required to remove and replace a pressure tube.
Attempting to provide in-situ sampling (without pressure tube removal) presents numerous difficulties. Obtaining a useful sample is made difficult by the hard oxidized surface, and the need to obtain sample material from beneath the surface layer. To preserve the structural integrity of the tube and avoid detrimental residual stress, the sampling depth must be controlled and sampled region must be left with smooth changes in geometry in all axes. Furthermore, the technique used for removing the surface material or sample must not involve excessive heating, as this affects the results of the subsequent analysis. Another difficulty is the recovery of the sample for analysis and preventing particles from being left in the pressure tube. Also, it is desirable to have a simple sampling device that can operate in the presence of reactor coolant and high radiation fields.